J.H. Liang

Formation of tungsten silicide films by ion beam synthesis

Abstract In the present study, a MEVVA ion implanter was employed to implant tungsten ions into silicon wafers at an elevated temperature of 100°C. The acceleration voltage was 40 kV and the charge states of the implanted tungsten ions were 1+ (8%), 2+ (34%), 3+ (36%), 4+ (19%), and 5+ (3%). The ion fluences were 1×1017 and 3×1017 ions/cm2. The as-implanted specimens were furnace annealed in Ar under various temperatures for 30 min. The tungsten silicide film was analyzed by secondary ion mass spectroscopy, X-ray diffraction, cross-sectional transmission electron microscopy, Rutherford backscattering spectroscopy, and a four-point probe. The results indicate that the tungsten silicide film thickness is approximately 30–40 nm, depending on annealing temperature as well as ion fluence. The sheet resistance of the tungsten silicide film closely correlates to the depth profile of ion-implanted tungsten, the Si/W ratio, the crystallographic structure, and the microstructure of the tungsten silicide film. The maximum sheet resistance was obtained at annealing temperatures of 400°C and 550°C for ion fluences of 1×1017 and 3×1017 ions/cm2, respectively, while the minimum sheet resistance was obtained at annealing temperatures above 800°C for both ion fluences. The hexagonal crystallization phase of tungsten silicide, existing at annealing temperatures between 400 and 550°C and leading to smaller mean depth of the synthesized layer, can be clearly observed only at 3×1017 ions/cm2. The tetragonal crystallization phase of tungsten silicide, starting to form at an annealing temperature of 550°C and resulting in larger mean depth of the synthesized layer, is clearly observed at annealing temperatures above 800°C for both ion fluences. The microstructure of the as-implanted tungsten silicide film possesses amorphous and non-continuous properties as well as a rough surface. However, surface roughness can be markedly improved and a continuous and epitaxial layer of tungsten silicide can be obtained when the annealing temperature is increased to 800°C or higher.

Range parameters of bismuth ions in polystyrene

Abstract In this study, the Rutherford-backscattering spectrometry was used with a 2 MeV He + ion beam to measure the projected ranges and range stragglings of bismuth ions implanted into polystyrene in the energy range of 40–360 keV. Range-projection equations following the modified Biersack model and including the first four moments of the nuclear and the electronic energy losses were also employed to perform the theoretical calculations. The feasibility of including the finite maximum-impact parameter and the inelasticity effect in simulating nuclear stopping processes was considered, as well as the surface effect in simulating a finite target medium. Experimental results demonstrate that measurements closely correspond to theoretical values, with average differences of 9% for projected ranges and 21% for range stragglings.

Higher moments of the implanted-ion profiles of bismuth in silicon

Abstract In this paper, silicon wafers were implanted with bismuth ions ranging from 40 to 360 keV at room temperature. Secondary ion mass spectrometry was used to measure the implanted-ion profiles of bismuth in silicon. The first four moments of the implanted-ion profiles were experimentally determined through least-squares fitting the measured implanted-ion profiles to Pearson distributions. An extension of the Biersack theory up to the fourth order was combined with the Gibbons parabolic fitting formula in making theoretical predictions. A comparison of the measured with calculated values revealed a significant correlation between the calculated and measured values of both the lower (projected range and range straggling) and higher (skewness and kurtosis) moments of the implanted-ion profiles. In addition, the critical energy at which skewness equals zero is defined in this paper. An investigation of ion implantations over the whole range of incident ions reveals that skewness is negative if the incident-ion energy is greater than the critical energy, and vice versa. In addition, the depth profile of lighter ion implantation skews toward the target’s surface at lower incident-ion energies than with heavier ion implantation.

Transversal range straggling of 40 keV–2 MeV tungsten ions implanted in SiO2

Abstract This paper presents an experimental investigation of transversal range straggling of tungsten ions implanted in SiO 2 for a wide incident-ion-energy level of 40 keV–2 MeV. Tungsten ions were implanted into SiO 2 films tilted by angles of 7° and 55° with respect to the axis normal to the specimen for each incident-ion energy. The depth profiles of implanted tungsten ions were measured and fitted by using secondary ion mass spectrometry (SIMS) and Gaussian distribution, respectively. The measured values of projected range ( R p ), longitudinal range straggling ( ΔR p ) and transversal range straggling ( ΔR t ) were determined from the corresponding Gaussian-fitted distributions of 7° and 55° implants along with Furukawa and Matsumuras method. The calculated values of R p , ΔR p and ΔR t yielded by the SRIM Monte-Carlo simulation code are also presented herein for comparison. It was found that the calculated values of R p , ΔR p and ΔR t agreed to the corresponding measured values within (on average) 19%, 21% and 14%, respectively. The discrepancies between the calculated and measured values of both ΔR p and ΔR t become significant at rather low incident-ion energies.